WO2005112149A1

WO2005112149A1 - Negative electrode for nonaqueous electrolyte secondary battery

Info

Publication number: WO2005112149A1
Application number: PCT/JP2005/001500
Authority: WO
Inventors: Shinichi Musha; Hitohiko Honda; Masahiro Hyakutake; Kiyotaka Yasuda
Original assignee: Mitsui Mining & Smelting Co., Ltd.
Priority date: 2004-05-19
Filing date: 2005-02-02
Publication date: 2005-11-24
Also published as: JP2006004903A; JP3742828B2

Abstract

A negative electrode (10) comprising a pair of current collecting surface layers (4a, 4b) respectively serving as the front and rear surface, and an active material layer (3) interposed between the surface layers. The surface layers (4a, 4b) contain an element (6) having low lithium compound-forming power, and an element (5) having high lithium compound-forming power. They have no thick film conductor for current collection. The element (5) having high lithium compound-forming power contained in the surface layers (4a, 4b) is pulverized due to the charge/discharge, thereby causing cracks to cause cracking in the surface layers. The cracks propagate in the thickness direction of the surface layers and reach the active material layer (3), thereby forming many micropores (7) which enable permeation of the electrolyte.

Description

Specification

Negative electrode for non-aqueous electrolyte secondary battery

Technical field

The present invention relates to a negative electrode for a non-aqueous electrolyte secondary battery. More specifically, a negative electrode capable of obtaining a non-aqueous electrolyte secondary battery having a high current collecting property, preventing fall of an active material due to absorption and desorption of lithium ions, improving cycle life, and further increasing energy density. About.

Background art

[0002] In a negative electrode for a lithium ion secondary battery, an active material layer having a metal force such as tin, which is alloyed with Li, is provided on a current collector layer having a metal force which does not alloy with Li such as copper. It has been proposed to provide a surface coating layer made of a metal that does not alloy with Li or a surface coating layer made of an alloy of a metal that does not alloy with Li and a metal that alloys with Li on the active material layer (Patent Reference 1). According to this patent document, the reaction between the surface of the active material layer and the electrolytic solution can be suppressed by the surface coating layer, so that the deterioration of the active material layer surface can be suppressed, and the charge / discharge cycle characteristics can be improved. Being able to improve! /

[0003] Since the surface coating layer of this electrode only covers the active material layer in a thin island shape, a considerable area of the active material layer is exposed on the surface of the electrode and comes into direct contact with the electrolyte. ing. Therefore, the active material is liable to lose its electrode force due to expansion and contraction of the active material due to the absorption and desorption of Li accompanying charge and discharge.

[0004] Patent Document 1: US2002Z0168572A1

Disclosure of the invention

[0005] Accordingly, an object of the present invention is to provide a negative electrode for a non-aqueous electrolyte secondary battery that can solve the above-described various disadvantages of the related art.

[0006] The present invention includes a pair of front and back current collecting surface layers which are in contact with an electrolytic solution and have conductivity, and have a high lithium compound forming ability and active material particles interposed between the surface layers. An active material layer, at least one surface layer of which is composed of an element having a low ability to form a lithium compound and an element and an element having a high ability to form a lithium compound. The object has been achieved by providing a negative electrode for a non-aqueous electrolyte secondary battery, which has no body.

Brief Description of Drawings

FIG. 1 is a schematic diagram showing a cross-sectional structure of one embodiment of a negative electrode of the present invention.

[FIG. 2] FIG. 2 (a) -FIG. 2 (c) are schematic views showing an enlarged main part of the negative electrode shown in FIG.

3 (a) to 3 (f) are step diagrams showing an example of a method for producing the negative electrode shown in FIG. 1.

FIG. 4 is a schematic diagram showing a cross-sectional structure of another embodiment of the negative electrode of the present invention.

FIG. 5 (a) to FIG. 5 (i) are process diagrams showing an example of a method for producing the negative electrode shown in FIG.

FIG. 6 is a schematic diagram showing a state in which a surface layer and fine voids are formed.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the present invention will be described based on preferred embodiments with reference to the drawings. FIG. 1 shows a schematic diagram of one embodiment of the negative electrode of the present invention. The negative electrode 10 of the present embodiment has a first surface la and a second surface lb that are a pair of front and back surfaces that are in contact with the electrolytic solution. The electrode 10 has an active material layer 3 containing active material particles 2 between both surfaces. The active material layer 3 is continuously covered by a pair of current collecting surface layers 4 a and 4 b formed on each surface of the active material layer 3. Each surface layer 4a, 4b includes a first surface la and a second surface lb, respectively. As is clear from FIG. 1, the electrode 10 has a current collector thick film conductor (for example, a metal foil with a thickness of about 8 mm—expanded metal) called a current collector used in conventional electrodes. I haven't.

[0009] The current collecting surface layers 4a and 4b have a current collecting function in the negative electrode 10 of the present embodiment.

The surface layers 4a and 4b are also used to prevent the active material contained in the active material layer 3 from dropping due to expansion and Z or contraction due to an electrode reaction.

[0010] Each of the surface layers 4a and 4b is thinner than a thick-film conductor for current collection used in a conventional electrode. Specifically, it is preferably a thin layer of about 0.3 to 20 m, particularly about 0.3 to 10 m, especially about 0.5 to 5 m. Thereby, the active material layer 3 can be coated almost uniformly and continuously with a minimum thickness. As a result, it is possible to prevent the particles 2 of the active material from falling off. In addition, by using such a thin layer and having a thick-film conductor for current collection, the ratio of the active material in the entire negative electrode is relatively high. Thus, the energy density per unit volume and per unit weight can be increased. In the conventional electrode, the ratio of the thick film conductor for current collection to the entire electrode was high, so there was a limit in increasing the energy density. The thin surface layers 4a and 4b in the above range are preferably formed by electrolytic plating as described later. The two surface layers 4a, 4b may have the same thickness or different thicknesses. The constituent materials of the two surface layers 4a and 4b may be the same or different, but at least one of the surface layers has a low ability to form a lithium compound, It must be composed of elements that have high formability. When one surface layer is composed of an element having a low ability to form a lithium compound and an element having a high ability to form a lithium compound, the other surface layer has an ability to form a lithium compound. Preferably, the elemental strength is low. Preferably, the two surface layers 4a and 4b contain an element having a low ability to form a lithium compound and an element having a high ability to form a lithium compound.

[0011] As described above, the two surface layers 4a and 4b include the first surface la and the second surface lb, respectively. When the negative electrode 10 of the present embodiment is incorporated in a battery, the first surface la and the second surface lb are in contact with the electrolyte. In contrast, a conventional thick-film current collector for an electrode does not come into contact with the electrolyte when an active material layer is formed on both surfaces, and the active material layer is formed on one surface. Even if formed, only one surface is in contact with the electrolyte. In other words, the negative electrode 10 of the present embodiment does not have the thick film conductor for current collection used in the conventional electrode, and the layer located on the outermost surface of the electrode, that is, the surface layers 4a and 4b have a current collection function. And also functions to prevent the active material from falling off.

[0012] Since each of the surface layers 4a and 4b including the first surface la and the second surface lb has a current collecting function, when the negative electrode 10 of the present embodiment is incorporated in a battery, There is an advantage that a lead wire for extracting current can be connected to any of the surface layers 4a and 4b.

[0013] As described above, at least one of the surface layers 4a and 4b is configured to include an element having a low ability to form a lithium compound and an element having a high ability to form a lithium compound. Since each of the surface layers 4a and 4b has such a configuration, the negative electrode 10 of the present embodiment has the following advantages. This advantage will be described with reference to FIG. FIG. 2 is a schematic diagram showing an enlarged main part of the negative electrode 10 of the present embodiment. When the negative electrode 10 of the present embodiment is incorporated in a nonaqueous electrolyte secondary battery and charged and discharged, the lithium compound contained in each of the surface layers 4a and 4b is formed as shown in FIG. 2 (a). The highly active element 5 gradually becomes finer and finer due to the absorption and desorption of lithium. On the other hand, the element 6 having a low ability to form a lithium compound contained in each of the surface layers 4a and 4b does not change even when charging and discharging are performed.

[0015] When the ability to form a lithium compound is high and the pulverization of the element 5 proceeds, cracks are formed on the surfaces of the surface layers 4a and 4b as shown in Fig. 2 (b). Then, the portion where the crack has occurred becomes a number of fine voids 7.

As the crack progresses, a fine void 7 grows in a layer, and as shown in FIG. 2 (c), the fine void 7 extends in the thickness direction of at least one of the surface layers 4a, 4b to form an active material layer. Reach 3 The minute gap 7 extends while bending. In FIG. 2 (c), all the formed fine voids 7 are drawn so as to reach the active material layer 3, but this is exaggerated for easy understanding of the present invention. In fact, not all the micro voids 7 reach the active material layer 3.

As a result of the formation of such fine voids 7, the electrolyte can sufficiently penetrate into the active material layer 3, and the reaction with the active material particles 2 sufficiently occurs. Since the surface tension of the non-aqueous electrolyte is smaller than that of the aqueous electrolyte, the non-aqueous electrolyte can sufficiently penetrate into the active material layer 3 even if the width of the fine voids 7 is small. As a result of the formation of the fine voids 7 in this manner, in the negative electrode 10 of the present embodiment, it is possible to prevent the particles 2 of the active material from falling off while ensuring the penetration of the electrolytic solution into the active material layer 3. .

Since the reaction in the negative electrode 10 occurs mainly on the surface facing the counter electrode, it is sufficient that the fine voids 7 are formed in at least one of the pair of surface layers 4a and 4b. However, in practical batteries, separators and counter electrodes are often arranged on both sides of the negative electrode! When the negative electrode 10 of the present embodiment is applied to such a battery, it is preferable that the fine voids 7 are formed in both the pair of surface layers 4a and 4b. When using the negative electrode 10 in which fine voids are formed only in one surface layer of the pair of surface layers 4a and 4b, a set of such negative electrodes 10 is prepared, and the fine voids in each of the negative electrodes 10 and 10 are reduced. By using the non-formed side surface layers facing each other and overlapping, micro voids 5 are formed in both of the pair of surface layers. The same effect as the negative electrode 10 can be obtained. In particular, a negative electrode having a configuration in which the surface layers of the negative electrodes 10 and 10 on which the fine voids are not formed are opposed to each other, a conductive metal foil is sandwiched between the negative electrodes 10 and 10, and the three members are joined and integrated. Is preferred. This is because such a configuration further improves the strength of the entire negative electrode. This is advantageous from the viewpoint of ensuring the strength of the negative electrode against bending stress when it is necessary to bend the negative electrode when producing a battery. As the conductive metal foil, various materials made of materials having low lithium-forming ability, such as an electrolytic copper foil, a rolled copper alloy foil, and a stainless steel foil, can be used. In view of the balance between the effect of improving strength and the energy density, the thickness is preferably about 5 to 35 m, particularly preferably 12 to 18 / zm. Further, as the conductive metal foil, a porous metal such as a punched metal, or a foil in which a lithium layer is formed on the surface of the various foils exemplified above can be used.

[0019] The size (pore diameter, length) of the fine void 7 changes depending on the number of times of charging and discharging of the battery. Therefore, the size of the fine void 7 cannot be uniquely determined. However, as a result of the study by the present inventors, when the surface layers 4a and 4b are viewed in a plan view by an electron microscope, the opening area of the fine voids 7 is 0.1 to 100 μm on average.

Especially if 1 one 30 mu m ² approximately, while ensuring adequate permeation of the electrolytic solution, it has been found that can effectively prevent detachment of the particles 2 in the active material. Also, when the surface layers 4a and 4b are viewed in plan by electron microscopy, no matter what field of view is taken, there are 1120,000 pieces, especially 10-1000 pieces, within a lcm X lcm square field of view. In particular, it was found that it is preferable that 30 to 500 microvoids 7 exist (this value is referred to as a distribution ratio). Further, when the surface layers 4a and 4b are viewed in plan by electron microscopy, the ratio of the total area of the pores of the microvoids 7 to the area of the observation visual field (this ratio is called the aperture ratio) is 0.1-10. %, Especially 11-5%, has been found to be preferred. The fine voids 7 have a width of about 0.1 to 100 / zm when the surface layers 4a and 4b are observed in cross section. U / zm, in particular, preferably as fine as about 0.1-10 μm.

[0020] In order to form a desired fine void, the ratio of the element 5 to the element 5 having a high ability to form a lithium compound contained in each of the surface layers 4a and 4b, and the element 6 being low! As a result of the study of the present inventors, it has been found that it is sufficient to set the range as appropriate. Specifically, each surface layer 4a And 4b, the element 6 having a low ability to form a lithium compound preferably accounts for 50% by weight or more. In particular, each of the surface layers 4a and 4b preferably contains 50 to 99.9% by weight of an element having a low ability to form a lithium compound and 0.1 to 50% by weight of an element having a high ability to form a lithium compound. In particular, it is particularly preferable that the composition contains 70 to 99% by weight of an element having a low ability to form a lithium compound and 110 to 30% by weight of an element having a high ability to form a lithium compound.

[0021] As the element 6, which has a low ability to form a lithium compound, nickel, copper, iron, cobalt, or an alloy thereof is preferably used. In particular, it is preferable to use a nickel tungsten alloy because the surface layer 4 can have high strength. On the other hand, as the element 5 having a high ability to form a lithium compound, tin, zinc, aluminum or indium is preferably used. These elements can be used alone or in combination of two or more.

Returning to FIG. 1, the active material layer 3 located between the two surface layers 4a and 4b contains the active material particles 2 having a high ability to form a lithium compound. Examples of the active material include a silicon-based material, a tin-based material, an aluminum-based material, and a germanium-based material. Particularly, a silicon-based material is preferable. Since the active material layer 3 is covered with the two surface layers 4a and 4b, the active material is effectively prevented from falling off due to absorption and desorption of lithium ions. In addition, for the reason described later, the active material particles 2 can come into contact with the electrolytic solution, so that the electrode reaction is not hindered.

The maximum particle size of the active material particles 2 is preferably 50 m or less, and more preferably 20 μm or less. When the particle size is represented by the D value, it is 0.1 to 8 m, especially 1

It is preferably 50 to 5 m. If the maximum particle size is more than 50 m, particles may easily fall off and the life of the electrode may be shortened. There is no particular limitation on the lower limit of the particle size, and the smaller the value, the better. In view of the method for producing the particles, the lower limit is about 0.01 m. The particle size of the particles is measured by laser-diffraction scattering type particle size distribution measurement and observation with an electron microscope (SEM observation).

[0024] When the amount of the active material relative to the entire negative electrode is too small, the energy density of the battery is not sufficiently improved, whereas when it is too large, the active material tends to fall off. In consideration of these, the amount of the active material is preferably 5 to 80% by weight based on the whole negative electrode, and more preferably. Preferably it is 10-50% by weight, more preferably 20-50% by weight.

[0025] The thickness of the active material layer 3 can be appropriately adjusted according to the ratio of the amount of the active material to the entire negative electrode and the particle size of the active material, and is not particularly critical in the present embodiment. Generally, it is about 100 m, especially about 3-40 m. The active material layer is preferably formed by applying a conductive slurry containing particles of the active material, as described later.

[0026] In the active material layer 3, it is preferable that a metal material having a low ability to form a lithium compound penetrates between particles contained in the layer. Preferably, the metal material penetrates over the entire region of the active material layer 3 in the thickness direction. Preferably, the active material particles 2 are present in the permeated metal material. That is, it is preferable that the particles 2 of the active material are not exposed on the surface of the negative electrode 10 and are embedded in the surface layers 4a and 4b. As a result, the adhesion between the active material layer 3 and the surface layers 4a and 4b is strengthened, and the falling of the active material particles 2 is further prevented. In addition, since electron conductivity is ensured between the surface layers 4a and 4b and the particles 2 of the active material through the material penetrated into the active material layer 3, an electrically isolated active material is generated. The generation of an electrically isolated active material in the deep part of the active material layer 3 is effectively prevented, and the current collecting function is maintained. As a result, a decrease in the function as the negative electrode is suppressed. Further, the life of the negative electrode can be prolonged. This is particularly advantageous when a semiconductor is used as the active material and has poor electron conductivity, such as a silicon-based material.

[0027] The metal material having a low ability to form a lithium compound penetrating into the active material layer 3 has at least a low ability to form a lithium compound contained in at least one surface layer, and may be of the same kind as the metal material. They may be different or different. For example, (a) when each surface layer contains a metal material having a low ability to form a lithium compound, the metal material may be the same as the metal material having a low ability to form a lithium compound that has penetrated into the active material layer 3. Good. In this case, since each material is the same, there is an advantage that a manufacturing method described later does not become complicated. Or (mouth) Each surface layer contains a metal material having a low ability to form a lithium compound, and the metal material penetrates into the active material layer 3 and has a low ability to form a lithium compound. They may be different. Furthermore, (c) each surface layer contains a metal material having a low ability to form a lithium compound, and both of the metal materials are in the form of a lithium compound that has penetrated into the active material layer 3. Low in performance, unlike metal materials.

[0028] In the case of (c), the metal compounds may be the same or different or different in the ability to form a lithium compound contained in each surface layer. That is, (i) the formation of the lithium compound which is the same as the metal material having low ability to form a lithium compound contained in each surface layer and which is infiltrated into the active material layer 3 by the metal material! Noh!場合 If different from the metal material, and (ii) the lithium material contained in each surface layer that has a low ability to form a lithium compound is different, and any of the metal materials penetrates into the active material layer 3 and Low in forming ability and may be different from metal materials.

It is preferable that the metal material having a low ability to form a lithium compound penetrating into the active material layer 3 penetrates the active material layer 3 in the thickness direction and is connected to both surface layers 4a and 4b. . As a result, the two surface layers 4a and 4b become electrically conductive through the material, and the electron conductivity of the entire negative electrode is further increased. That is, the negative electrode 10 of the present embodiment as a whole has a current collecting function. Low ability to form lithium compounds!ヽ It can be confirmed by electron microscope mapping that the metal material permeates all over the active material layer 3 in the thickness direction and the two surface layers 4a and 4b are connected to each other. . A preferred method for infiltrating the active material layer 3 with a metal material having a low ability to form a lithium compound will be described later.

[0030] The gaps 8 between the active material particles 2 in the active material layer 3 may not be completely filled with a metal material having a low ability to form a lithium compound. It is preferable to note that this gap is different from the fine gap 7 formed in the current collecting surface layers 4a and 4b. Due to the presence of the void 8, the stress caused by the active material particles 2 expanding and contracting by absorbing and desorbing lithium is reduced. From this viewpoint, the ratio of the voids 8 in the active material layer 3 is preferably about 5 to 30% by volume, particularly preferably about 5 to 9% by volume. The ratio of the voids 8 can be determined by electron microscope mapping. As will be described later, since the active material layer 3 is formed by applying and drying a conductive slurry containing the active material particles 2, the voids 8 are naturally formed in the active material layer 3. Therefore, in order to keep the ratio of the voids 8 in the above range, for example, the particle size of the particles 2 of the active material, the composition of the conductive slurry, and the application conditions of the slurry may be appropriately selected. Also apply slurry and dry After drying to form the active material layer 3, the ratio of the voids 8 is adjusted by pressing under appropriate conditions.

The active material layer 3 preferably contains a conductive carbon material in addition to the active material particles 2. Thereby, the electron conductivity is further imparted to the negative electrode 10. From this viewpoint, the amount of the conductive carbon material contained in the active material layer 3 is preferably 0.1 to 20% by weight, particularly preferably 110 to 10% by weight. As the conductive carbon material, for example, particles such as acetylene black and graphite are used. The particle size of these particles is preferably 40 μm or less, particularly preferably 20 μm or less from the viewpoint of further imparting electron conductivity. There is no particular limitation on the lower limit of the particle size of the particles, and a smaller value is more preferable. In view of the method for producing the particles, the lower limit is about 0.01 μm.

Next, a preferred method for manufacturing the negative electrode 10 of the present embodiment will be described with reference to FIG.

First, a carrier foil 11 is prepared as shown in FIG. The material of the carrier foil 11 is not particularly limited. For example, a copper foil can be used. Since the carrier foil 11 is used as a support for producing the negative electrode 10, it is preferable that the carrier foil 11 has such a strength that no swelling or the like occurs in the production process. Therefore, the thickness of the carrier foil 11 is preferably about 10 to 50 m. As described above, an important role of the carrier foil 11 is a support for manufacturing the negative electrode 10. Therefore, when the strength of the surface layer 4 is sufficient, it is not always necessary to manufacture the negative electrode 10 using the carrier foil.

[0033] The carrier foil 11 is preferably manufactured by electrolysis. Specifically, using a rotating drum as a cathode, electrolysis is performed in an electrolytic bath containing metal ions such as copper to deposit metal on the peripheral surface of the drum. Carrier foil 11 is obtained by peeling the deposited metal around the drum.

Next, as shown in FIG. 3B, a thin release layer 12 is formed on one surface of the carrier foil 11. When the carrier foil 11 is manufactured by electrolysis, one surface of the carrier foil 11 has a smooth glossy surface, and the other surface has a matte surface with irregularities. The glossy surface is the surface facing the drum peripheral surface, and the matte surface is the deposition surface. In the present production method, the release layer 12 may be formed on either the glossy surface or the matte surface.

The release layer 12 is preferably formed by, for example, chrome plating, nickel plating, lead plating, chromate treatment, or the like. The reason for this is that the surface of the release layer 12 An oxide or acid salt layer is formed on the surface, and this layer has a function of reducing the adhesion between the carrier foil 11 and an electrolytic plating layer described later and improving the releasability. Further, an organic compound can be used as the release agent. In particular, it is preferable to use a nitrogen-containing compound or a sulfur-containing compound. As the nitrogen-containing compound, for example, benzotriazole (

BTA), carboxybenzotriazole (CBTA), tolyltriazole (ΤΤΑ), Ν ', N'-bis (benzotriazolylmethyl) urea (BTD-U) and 3-amino-1Η-1,2,4- Triazole-based compounds such as triazole (ATA) are preferably used. Sulfur-containing compounds include mercaptobenzothiazole (MBT), thiocyanuric acid (TCA), and 2-benzimidazolthiol (BIT). These organic compounds are used after being dissolved in alcohol, water, an acidic solvent, an alkaline solvent or the like. For example, when CBTA is used, its concentration is preferably 2-5 gZl. In order to form the release layer 12 made of an organic compound, a dipping method can be adopted in addition to the coating method. The thickness of the peeling layer 12 is preferably 0.05-3 / zm, and the point force capable of performing peeling successfully is also preferable. The surface roughness Ra of the release layer 12 after the release layer 12 is formed is 0.01 to 3 m, particularly 0.01 to 1 m, as in the case where the active material layer 3 is directly formed on the carrier foil 11. 1 / ζ πι, especially 0.01-0.2 m force ^ preferred

After the release layer 12 is formed, as shown in FIG. 3 (c), the surface layer 4b containing the element having a low ability to form a lithium compound and the element and the element having a high ability to form a lithium compound is formed on the separation layer 12 as shown in FIG. To form There are no particular restrictions on the method of forming the surface layer 4b, and various known thin film forming methods can be employed. For example, methods such as electroplating, sputtering, chemical vapor deposition, and physical vapor deposition can be used. A particularly preferred forming method is electrolytic plating.

Subsequently, as shown in FIG. 3D, a conductive slurry containing particles of the active material is applied on the surface layer 4b to form the active material layer 3. In this case, if the surface layer 4b is formed by electroplating, the surface of the surface layer 4b to which the conductive slurry is applied is a deposition surface, that is, a mat surface, and the surface roughness is high. By applying a conductive slurry to the surface of the surface layer 4b in such a surface state, there is an advantage that the adhesion between the particles of the active material and the surface layer 4b is improved.

[0038] The slurry contains particles of the active material, particles of the conductive carbon material, a binder and a diluting solvent. Contains. Among these components, the particles of the active material and the particles of the conductive carbon material are as described above. Styrene butadiene rubber (SBR), polyvinylidene fluoride (PVDF), polyethylene (PE), ethylene propylene diene monomer (EPDM), etc. are used as the binder. As a diluting solvent, N-methylpyrrolidone, cyclohexane and the like are used. The amount of the active material particles in the slurry is preferably about 14 to 40% by weight. The amount of the conductive carbon material particles is preferably about 0.4 to 4% by weight. The amount of the binder is preferably about 0.4-4% by weight. A slurry is prepared by adding a diluting solvent to these components.

The active material layer 3 formed by drying the slurry has many minute spaces between particles. After the active material layer 3 is formed, as shown in FIG. 3 (e), the ability to form a lithium compound is low, the ability to form elements and lithium compounds is high, To form a surface layer 4a containing The method for forming the surface layer 4a can be the same as the method for forming the surface layer 4b described above. In particular, in order for the material constituting the surface layer 4a to penetrate throughout the thickness direction of the active material layer 3 so that the surface layers 4a and 4b are electrically connected, the surface layer 4a is electrolytically plated. It is advantageous to form That is, it is advantageous to form at least one surface layer by electrolytic plating (hereinafter, this electrolytic plating is also referred to as permeation plating). By dipping the carrier foil 11 on which the active material layer 3 is formed into a plating bath, a plating solution enters the minute space in the active material layer 3 and an interface between the active material layer 3 and the surface layer 4b. , And electroplating is performed under this condition. The conditions of the penetration plating conditions are important for depositing a low-metallic material having a low ability to form a lithium compound into the active material layer 3. For example, when the surface layer 4a is formed by electroplating, the surface layer 4a is made of, for example, copper (element having a low lithium compound forming ability, element) and tin (high element having a lithium compound forming ability, element). Are as follows. As the plating bath, those having a composition in an ordinary literature, generally a cyan bath, a pyrophosphate bath, a borofluoride bath and the like can be used. Among these, an example of the bath composition of a cyanation bath and electrolysis conditions are shown below.

• Na SnO · 2Η O: 60g / l

2 3 2

• NaCN: 25g / l 'Current density: 5AZdm ²

• Bath temperature: 65 ° C

When this plating bath is used, copper (a low lithium compound forming ability! Element) and tin (a high lithium compound forming ability element) and tin (element having a high lithium compound forming ability) are interposed between active material particles in the active material layer 3 due to permeation plating. Precipitates. Further, a surface layer 4a containing copper and tin as constituent materials is formed on the active material layer 3.

[0040] The surface layer may be made of, for example, nickel (a low-element capable of forming a lithium compound) and silicon.

(Element with a high ability to form lithium compounds, elements) When the composition is used, the particles of silicon oxide are suspended in an electrolytic plating solution containing nickel, and the plating solution is subjected to electrolytic plating while stirring. By incorporating silicon into the layer, a desired surface layer can be obtained. In this case, only nickel (element having a low ability to form a lithium compound, an element) precipitates between the particles of the active material in the active material layer 3 due to permeation.

Finally, as shown in FIG. 3 (f), the carrier foil 11 is separated from the surface layer 4b by separation. Thereby, the negative electrode 10 is obtained. At the time of peeling, the peeling layer 12 may remain on the carrier foil 11 side or the surface layer 4b side depending on the thickness and the kind of the peeling agent. Or it may remain in both. In any case, since the thickness of the release layer 12 is extremely small, the performance of the obtained negative electrode 10 is not affected at all.

[0042] In the above method, both surface layers 4a and 4b are configured to include an element having a low ability to form a lithium compound and an element having a high ability to form a lithium compound. Instead, one surface layer is composed of an element having a low ability to form a lithium compound and an element having a high ability to form a lithium compound, and the other surface layer is made of an element having a low ability to form a lithium compound. You can also In this case, two kinds of plating baths are used: a plating bath containing an element having a low ability to form a lithium compound and a lithium compound having a high ability to form a lithium compound, and a plating bath having a low ability to form a lithium compound and containing an element. If you do electrolysis plating.

Further, in the above-described method, two operations were performed simultaneously, namely, the permeation into the active material layer 3 and the operation of forming a surface layer on the surface of the active material layer 3. In this case, the metal material having a low ability to form a lithium compound deposited in the active material layer 3 and the metal material having a low ability to form a lithium compound contained in the surface layer are the same. Instead of these operations, two operations It may be performed separately. That is, after performing the operation of infiltrating the active material layer 3, the carrier foil 11 on which the active material layer 3 is formed is immersed in another plating bath, and the surface of the active material layer 3 is formed on the active material layer 3 by electrolytic plating. A layer may be formed. By performing this operation, a metal material having a low ability to form a lithium compound contained in each surface layer and a metal material having a low ability to form a lithium compound deposited in the active material layer 3 are made different from each other. can do.

According to the present production method, a negative electrode in which both surfaces of the negative electrode can be used for the electrode reaction can be obtained by performing the operation of forming the active material layer only once. In the conventional negative electrode, in order to use both surfaces of the electrode for the electrode reaction, it was necessary to form active material layers on both surfaces of the current collector. In other words, the operation of forming the active material layer had to be performed twice. Therefore, according to the present production method, the production efficiency of the negative electrode is extremely improved. Further, according to the present manufacturing method, the carrier foil 11 is not peeled off until the negative electrode 10 is incorporated into the battery, and the carrier foil 11 is peeled off immediately before the negative electrode 10 is assembled. There is also an advantage that 10 can be transported with good handling.

As another method of the present manufacturing method, the active material layer 3 is directly formed on the carrier foil 11 on which the release layer 12 is formed without forming the surface layer 4b, and then the entire active material layer 3 is formed. By electroplating, the surface layer 4b can be formed between the active material layer 3 and the carrier foil 11, and the surface layer 4a can be formed on the active material layer 3. In this case, the conditions for electroplating can be the same as those described above. According to this alternative method, since each surface layer can be formed by an operation of performing only one electroplating, the production of the negative electrode 10 can be efficiently performed.

The negative electrode 10 thus obtained is used together with a known positive electrode, separator and non-aqueous electrolyte to form a non-aqueous electrolyte secondary battery. The positive electrode is prepared by suspending a positive electrode active material and, if necessary, a conductive agent and a binder in an appropriate solvent to prepare a positive electrode mixture, applying the mixture to a current collector, drying the mixture, rolling, pressing, and Obtained by cutting and punching. As the positive electrode active material, a conventionally known positive electrode active material such as a lithium nickel composite oxide, a lithium manganese composite oxide, a lithium cobalt composite oxide, and the like are used. As the separator, a synthetic resin nonwoven fabric, a polyethylene or polypropylene porous film, or the like is preferably used. In the case of a lithium secondary battery, the non-aqueous electrolyte is lithium supporting electrolyte. It consists of a solution in which organic salts are dissolved in an organic solvent. As the lithium salt, for example, LiBF, LiCl

Four

O, LiAlCl, LiPF, LiAsF, LiSbF, LiSCN, LiCl, LiBr ゝ Lil, LiCF SO, Li

4 4 6 6 6 3 3

C F SO is exemplified.

4 9 3

As another embodiment of the negative electrode of the present invention, as shown in FIG. 4, a negative electrode 10 ′ in which fine voids 7 are already formed in the surface layers 4 a and 4 b when the surface layers 4 a and 4 b are formed. No. In the negative electrode 10 ', fine gaps 7 have already been formed in the surface layers 4a and 4b before charging / discharging, and the fine gaps 7 grow by charge / discharge. FIG. 5 illustrates a method for manufacturing the negative electrode 10 ′ shown in FIG.

First, as shown in FIG. 5A, a carrier foil 11 is prepared. As the carrier foil 11, the same as that shown in FIG. 3A can be used. Next, as shown in FIG. 5 (b), a thin layer covering 13 made of a material different from the material constituting the surface layer 4b is formed on one surface of the carrier foil 11. Thereafter, a surface layer 4b is formed by electrolytic plating. By this operation, the fine voids 7 can be formed in the surface layer 4b, and the number of the fine voids 7 and the open area can be easily controlled.

[0049] The cover 13 is used to form a large number of fine voids 7 in the surface layer 4b by making the electron conductivity of the surface on which the surface layer 4b is formed nonuniform. It is preferable that the coating 13 is formed to have a thickness of ^ 0.001−, particularly 0.002−0.5 m, and particularly 0.005−0.2 m. This is because by setting the thickness to this level, the coating 13 covers the surface of the carrier foil 11 discontinuously, for example, in an island shape.

[0050] The cover 13 is made of a material different from the constituent material of the surface layer 4b. This allows the surface layer 4b to be successfully peeled from the carrier foil 11 in the peeling step. In particular, the coating 13 is a material different from the constituent material of the surface layer 4b and Cu, Ni, Co, Mn, Fe, Cr, Sn, Zn, In, Ag, Au, C, Al, Si, It is preferable that it is configured to include at least one element of Ti and Pd.

[0051] The method for forming the coating 13 is not particularly limited. For example, the method of forming the cover 13 can be selected in relation to the method of forming the surface layer 4b. Specifically, when the surface layer 4b is formed by electroplating, it is preferable that the cover 13 is also formed by electroplating in view of production efficiency and the like. However other methods such as electroless plating, sputtering, physical vapor deposition The coating 13 can be formed by a method, a chemical vapor deposition method, a sol-gel method or an ion plating method.

When the coating 13 is formed by electrolytic plating, an appropriate plating bath and plating conditions are selected according to the constituent material of the coating 13. For example, when the cover 13 is made of tin, a plating bath having the following composition or a tin bath having a composition shown below can be used. When this plating bath is used, the bath temperature is preferably about 15-30 ° C., and the current density is preferably about 0.5-10 OAZdm ² .

• Cresol sulfonic acid 70—lOOgZl

As described above, the cover 13 is used to make the electron conductivity of the surface on which the surface layer 4b is formed non-uniform. Therefore, if the electron conductivity of the constituent material of the cover 13 is significantly different from the electron conductivity of the carrier foil 11, the formation of the cover 13 immediately turns the surface of the surface layer 4b into an uneven state. Become. For example, there is a case in which rubber is used as a constituent material of the cover 13. On the other hand, when a material having the same electronic conductivity as the carrier foil 11, for example, various metal materials such as tin is used as a constituent material of the cover 13, the surface of the cover 13 may be changed depending on the formation of the cover 13. The electron conductivity of the surface on which the layer 4b is formed does not immediately become uneven. Therefore, when the covering 13 is formed from such a material, it is preferable to expose the carrier foil 11 on which the covering 13 is formed to an oxygen-containing atmosphere, for example, the atmosphere in a dry state. Thereby, the surface of the cover 13 (and the exposed surface of the carrier foil 11) is oxidized (see FIG. 5 (c)). By this operation, the electronic conductivity of the surface on which the surface layer 4b is formed becomes non-uniform. When electrolytic plating described below is performed in this state, a difference occurs in the deposition rate between the surface of the cover 13 and the exposed surface of the carrier foil 11, and the fine voids 7 can be easily formed. The degree of acidification is not critical in the present invention. For example, the present inventors have found that it is sufficient to leave the carrier foil 11 on which the cover 13 is formed in the air for about 10 to 30 minutes. However, forcible oxidation of the carrier foil 11 on which the coating 13 is formed is not prevented.

[0054] When the carrier foil 11 on which the coating 13 is formed is exposed to an oxygen-containing atmosphere, it is dried. The reason for this is to carry out acidification efficiently. Covered by electroplating, for example

When the carrier foil 13 is formed, the carrier foil 11 is removed from the plating bath, dried using a dryer or the like, and then left in the air for a predetermined time. When a dry method such as a sputtering method or various vapor deposition methods is used as a method for forming the coating 13, a drying operation is unnecessary, and after forming the coating 13, the coating 13 may be left as it is in the atmosphere.

After the covering 13 is oxidized, a release layer 12 is formed thereon as shown in FIG. 5 (d).

As the release agent, the same release agent as shown in FIG. 3 (b) can be used. The step of forming the peeling layer 12 is performed only in order to successfully peel the negative electrode 10 ′ from the carrier foil 11 in the peeling step (FIG. 5 (i)) described later. Therefore, even if this step is omitted, fine voids can be formed in the surface layer 4b. Next, as shown in FIG. 5 (e), after the release agent 12 is applied, the constituent material of the surface layer 4b is electrodeposited by electroplating to form the surface layer 4b. Many fine voids 7 are formed in the formed surface layer 4b. In FIG. 5 (e), the minute voids 7 are formed at the positions of the vertices of the covering 13, but this is for convenience, and actually, The minute void 7 is not always formed at the position of the vertex.

Next, as shown in FIG. 5 (f), a conductive slurry containing particles of the active material is applied on the surface layer 4b to form the active material layer 3. Regarding the composition of the conductive slurry and the formation of the active material layer 3, the description related to FIG.

After the active material layer 3 is formed, as shown in FIG. 5 (g), a coating liquid containing a conductive polymer is applied thereon and dried to form a coating film 14. The coating liquid is formed by dissolving a conductive polymer in a volatile organic solvent.

[0058] As the conductive polymer, a conventionally known polymer whose type is not particularly limited can be used. For example, poly (pyridene) fluoride (PVDF), polyethylene oxide (PEO), polyacryl-tolyl (PAN), polymethyl methacrylate (PMMA) and the like can be mentioned. In particular, it is preferable to use a lithium ion conductive polymer. Further, the conductive polymer is preferably a fluorine-containing conductive polymer. This is because the fluorine-containing polymer has high thermal and chemical stability and excellent mechanical strength. Considering these facts, we use poly (pyridene fluoride), a fluorine-containing polymer with lithium ion conductivity. Is particularly preferred.

As the organic solvent for dissolving the conductive polymer, for example, when polyvinylidene fluoride is used as the conductive polymer, N-methylpyrrolidone or the like can be used.

As described above, the coating liquid is applied on the active material layer 3. Since the active material layer 3 is a layer containing particles of the active material, its surface is uneven. Therefore, the coating liquid easily accumulates in the concave portions on the surface of the active material layer 3. When the solvent evaporates in this state, the thickness of the coating film 14 becomes uneven. In other words, the thickness of the coating film corresponding to the concave portion on the surface of the active material layer 3 is large, and the thickness of the coating film corresponding to the convex portion is small. In the present production method, a large number of fine voids are formed in the surface layer 4a by utilizing the unevenness of the thickness of the coating film 14.

[0061] In the present production method, the mechanism by which a large number of fine voids are formed in the surface layer 4a is considered as follows. The carrier foil 11 on which the coating film 12 is formed is subjected to electrolytic plating treatment, and the carrier foil 11 on which the coating film 14 is formed is subjected to electrolytic plating treatment, and is coated as shown in FIG. 5 (h). The surface layer 4a is formed on the film 14. This state is shown in FIG. 6 which is an enlarged view of a main part of FIG. 5 (h). The conductive polymer that forms the coating film 14 has electronic conductivity, though not so much as metal. Therefore, the coating 14 has different electron conductivity depending on its thickness. As a result, when a metal is deposited by electroplating on the coating film 14 containing a conductive polymer, a difference occurs in the deposition rate according to the electron conductivity, and the difference in the deposition rate causes the surface layer 4a to have a difference. Fine voids 7 are formed. In other words, a portion having a low electrodeposition rate, in other words, a thick portion of the coating film 14 is liable to become the fine void 7.

[0062] The pore size / existence density of the fine voids 7 can be controlled by the concentration of the conductive polymer contained in the coating liquid. For example, when the concentration of the conductive polymer is low, the pore size tends to decrease, and the existing density also tends to decrease. Conversely, when the concentration of the conductive polymer is high, the pore size tends to increase. From this viewpoint, the concentration of the conductive polymer in the coating liquid is preferably 0.05 to 5% by weight, particularly preferably 13 to 13% by weight. The conductive polymer can be applied on the active material layer 3 by a dipping method other than the coating method.

[0063] In this way, the negative electrode 10 'is obtained. Finally, as shown in FIG. 5 (i), the negative electrode 10 ′ is peeled off from the carrier foil 11. [0064] The present invention is not limited to the above embodiment. For example, in order to increase the reaction active point between the active material particles and the electrolytic solution, a hole is formed on at least one surface of the negative electrode by using a laser, a bunch, a needle, or the like and reaches at least a part of the active material layer. A hole or a through hole extending in the thickness direction of the negative electrode may be formed.

Further, the negative electrode of each of the above embodiments can be used alone, or a plurality of the negative electrodes can be used in an overlapping manner. In the latter case, a conductive foil (for example, a metal foil) serving as a core material can be interposed between adjacent negative electrodes.

In the above embodiment, the surface layers 4a and 4b have a single-layer structure. Alternatively, at least one of the surface layers may have a multilayer structure of two or more layers. For example, at least one surface layer is composed of nickel (low lithium compound forming ability, element) and lithium compound high forming ability, elemental lower layer, copper (low lithium compound forming ability, element) and lithium. By adopting a two-layer structure of an upper layer, which has a high ability to form a lithium compound and an elemental force, it is possible to more effectively prevent the negative electrode from being significantly deformed due to a change in the volume of the active material. When the surface layer has a multilayer structure, the ability to form a lithium compound contained in the surface layer is low, the ability to form a lithium compound penetrating at least one of the metal materials into the active material layer 3 is low, and the metal material is low. And different materials. Alternatively, any of the metal materials having a low ability to form a lithium compound contained in each surface layer may be a material having a low ability to form a lithium compound permeating the active material layer 3 and a material different from a metal material.

When the constituent material of the surface layer is different from the material penetrating into the active material layer 3, the material penetrating into the active material layer 3 is different from the material penetrating the active material layer 3 and the surface layer. May be present up to the boundary of. Alternatively, the material penetrating into the active material layer 3 may constitute a part of the surface layer beyond the boundary. Conversely, the constituent material of the surface layer may exist in the active material layer 3 beyond the boundary.

[0068] In addition, by performing an operation of depositing a material having a low ability to form a lithium compound in the active material layer 4 using two or more different plating baths, a metal deposited in the active material layer 4 is formed. The material can be two or more different multilayer structures.

In the manufacturing method shown in FIG. 5, the cover 13 is not formed when the surface layer 4b is formed, and instead, the above-described conductive film is placed on the carrier foil 11 (or the release layer 12). Poly By coating the coating solution containing the polymer and forming the surface layer 4b thereon, a large number of fine voids can be formed in the surface layer 4b. In this case, the surface roughness RaCFIS B 0601) of the carrier foil 11 should be 0.05 to 5 μm, particularly 0.2 to 0.8 μm. The fine voids having the desired diameter and existing density can be easily formed. Point power that can be formed.

Industrial applicability

As described above in detail, according to the negative electrode for a non-aqueous electrolyte secondary battery of the present invention, fine voids are formed in the surface layer for current collection by charging and discharging. The fine voids are large enough to prevent the electrolyte from penetrating and to prevent the active material particles from falling off. As a result, after the active material and the electrolytic solution are sufficiently in contact with each other, the active layer is prevented from falling off by the surface layer, and the current collecting property of the active material is ensured even when charging and discharging are repeated. Further, the secondary battery using this negative electrode has a low deterioration rate even if charge and discharge are repeated, greatly increases the cycle life, and increases the charge and discharge efficiency. Further, since the conductive metal foil layer as the core material, that is, the current collector used for the conventional negative electrode is not used, the ratio of the active material in the entire negative electrode can be higher than that of the conventional negative electrode. As a result, a secondary battery having a high energy density per unit volume and per unit weight can be obtained.

Claims

The scope of the claims

[1] A pair of front and back current collecting surface layers that are in contact with the electrolytic solution and have conductivity, and an active material layer containing active material particles having a high ability to form a lithium compound interposed between the surface layers. In addition, at least one surface layer includes an element having a low ability to form a lithium compound and an element having a high ability to form a lithium compound, and has a thick-film conductor for current collection.

V. A negative electrode for a non-aqueous electrolyte secondary battery, characterized in that:

[2] The surface layer is configured to contain 50-99.9% by weight of an element having a low ability to form a lithium compound and 0.1-50% by weight of an element having a high ability to form a lithium compound. 2. The negative electrode for a non-aqueous electrolyte secondary battery according to item 1.

[3] The element according to claim 1, wherein the element having a low ability to form a lithium compound is nickel, copper, iron or cobalt, and the element having a high ability to form a lithium compound is tin, zinc, aluminum or indium. Anode for water electrolyte secondary battery.

[4] The non-woven fabric according to claim 1, wherein at least one of the surface layers is formed with a large number of fine voids extending in the thickness direction thereof and reaching the active material layer, and through which an electrolyte solution can permeate. Anode for water electrolyte secondary battery.

[5] The fine voids are formed when the surface layer is formed, or have a high ability to form a lithium compound contained in the surface layer. 5. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 4, wherein the negative electrode is formed by forming a crack in the surface layer.

6. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the active material having a high ability to form a lithium compound is a silicon-based material.

[7] In the active material layer, the ability to form a lithium compound between particles of the active material is low, the metal material penetrates, and both surface layers are electrically connected, and the whole is collectively collected. 2. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 1, which has a function.

8. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein each of the surface layers has a thickness of 0.3 to 20 μm.

9. The negative electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the active material layer is formed by applying a conductive slurry containing particles of the active material.

[10] The method according to claim 1, wherein at least one of the surface layers is formed by electrolytic plating.

2. The negative electrode for a non-aqueous electrolyte secondary battery according to item 1.

[11] The metal material contained in the surface layer has a low ability to form a lithium compound, and the metal material penetrates into the active material layer and has a low ability to form a lithium compound. 2. The electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein

[12] The metal material, which is included in the surface layer and has a low ability to form a lithium compound and has a low ability to form a lithium compound and penetrates into the active material layer, is of the same type. 2. The electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein

[13] At least one of the surface layers has a multilayer structure of two or more layers, and at least one of constituent materials of each layer in the surface layer of the multilayer structure penetrates the active material layer to form a lithium compound. 2. The electrode for a non-aqueous electrolyte secondary battery according to claim 1, wherein the electrode is a material different from a metal material having low formability.